"....do I have to pay the taxes on the dividends she earned?"
Probably, because either the dividends were earned during the period of time you owned the stocks or they are considered income in respect of a decedent and you actually received the dividends.
Also make sure that you adjust the basis in the stocks appropriately. This can be significant for some stocks.
The basis in the stock you inherited is the FMV on the date of death unless the estate chose the alternate valuation date that's six months after the date of death. It is critical you maintain some type of record for this new basis. This would be fairly simple if an estate tax return was prepared, but this is not always the case.
Also make sure you contact shareholder services to make sure that the ownership of the stock gets updated on the company's records to now reflect you as the owner.
Basically, all payouts received after her passing are taxable income to the beneficiary recipient. It has nothing to do with when you actually inherited and got ownership of the stock. It has everything to do with the date of her passing.